This invention covers a tractor without drawbars for towing aircraft through its front steering wheels, which may be lifted or not. Several types of vehicles are known to tow airplanes, but they all have some drawbacks and defects. For instance, U.S. Pat. No. 2,987,133 illustrates a towing vehicle provided with a long rotating framework, the free end of which is fitted with two wheels driven by the framework and a system connecting the wheels of the aircraft to the shaft. The wheels rotating in touch with the aircraft cause the latter to move. This solution is rather complicated, its functioning is uncertain and requires complex coupling and hooking up operations.
Other solutions, such as U.S. Pat. No. 3,025,922 and DE-A-2826161 are known, consisting in a device like a motor-driven hoe, provided with a motorized roller in contact with one wheel of the airplane. Obviously, these solutions can only be used for moving small and light aircraft.
U.S. Pat. No. 3,005,510 is also known, which features a motor-driven belt to be inserted between the wheel sets of the aircraft. However, this solution can usually not be adopted on normal airliners.
Finally, EP-A-0156842 is known, grouping two original Finnish Patents FI-A-75310 and FI-A-78426. This document describes a trolley with a recessed central zone in which the lateral non steering wheel or wheels of the airplane may penetrate. This trolley is provided with mobile rollers, moving in transverse vertical as well as in horizontal lengthwise direction, so that they may be placed adjacent, in front of or behind the wheels of the aircraft. At least one of these rollers is motor-driven and will drive the non-steering wheels or set of lateral wheels of the plane.
But even this latter solution has some drawbacks. Since the trolley has no driving wheels, it has to be driven by an automotive vehicle. The rollers in touch with the lateral wheels of the aircraft are rather small, with little towing friction and there are no means to ensure a stable lateral support for the wheels of the airplane. These rollers are supported on one side only by the bearing frame so that they may get warped and deviated from their direction of motion. The above described towing device is applied to the lateral non-steering wheels of the airplane, so that the pilot has to steer the plane during towing or other persons shall move the front steering wheels of the aircraft with the aid of proper equipment.
Furthermore, the front and rear rollers are controlled by one single mechanism that will approach and grip the wheels of the airplane so that the docking manoeuvre takes much time and is difficult.
Recently, equipment described in the above mentioned EP-A-0156842 was mounted on a tractor, but the drawbacks still remain.
Recently the European Patent No. 93112100.8 has been filed covering a tractor to tow airplanes through its front steering wheels. The tractor is essentially provided with two motor-driven front rollers mounted on a transverse mobile axle, lengthwise to the tractor, while two more motor-driven rollers at the rear are revolving around a vertical axis from a central to a lateral position and vice-versa. The steering front wheels of the aircraft are gripped between these roller sets which, being motor-driven, will cause the aircraft wheels to rotate thus towing the aircraft without direct intervention of the tractor, the driving system of which is cut-out during towing.